SOIL-WATER DEPLETION BY SUNFLOWER AND SORGHUM UNDER RAIN-FED CONDITIONS

Grain sorghum [Sorghum bicolor (L) Moench ] and sunflower (Helianthus annuus L.), were grown under rainfed conditions in 1988 and 1989 near Manhattan, Kansas, on a Muir silt loam (Cumulic Haplustoll, fine-silty, mixed, mesic) to determine if the two crops exploited water at different depths and at different rates in the soil. Soil water content was measured weekly to a depth of 3.20 m with a neutron probe to ascertain depletion rate and depth. Transpiration, stomatal resistance, canopy temperature, and plant water potential also were analyzed to help explain any differences in soil water depletion that might be observed. Sunflower depleted water to a deeper depth, and had a higher rate of depletion at lower depths, than sorghum. In 1988 and 1989, sunflower depleted water to 2.55 and 2.70 m, respectively. About 90% of the total water used by sorghum was from the 0 to 1.65 m and 0 to 1.50 m depth in 1988 and 1989, respectively. Sunflower had a higher rate of depletion compared to sorghum below the 1.20 m depth ( 1 988) and the 1.65 m depth ( 1 989). Sunflower had a higher transpiration rate, lower stomatal resistance, cooler canopy temperature, and higher leaf water potential compared to sorghum, perhaps because, in part, sunflower could exploit more water at deeper soil depths. The results suggested that sunflower might be planted in rotations with shallow-rooted crops or after irrigated crops to take advantage of water at depth.